The Ames test is used to identify mutagenic, potentially carcinogenic chemicals. A
Salmonella histidine auxotroph is used as the test strain, exposed to a potential mutagen/carcinogen. The number of reversion mutants capable of growing in the absence of supplied histidine is counted and compared with the number of natural reversion mutants that arise in the absence of the potential mutagen.
What mutation is used as an indicator of mutation rate in the Ames test?
Why can the Ames test work as a test for carcinogenicity?
Key concepts and summary
A
mutation is a heritable change in DNA. A mutation may lead to a change in the amino-acid sequence of a protein, possibly affecting its function.
A
point mutation affects a single base pair. A point mutation may cause a
silent mutation if the mRNA codon codes for the same amino acid, a
missense mutation if the mRNA codon codes for a different amino acid, or a
nonsense mutation if the mRNA codon becomes a stop codon.
Missense mutations may retain function, depending on the chemistry of the new amino acid and its location in the protein. Nonsense mutations produce truncated and frequently nonfunctional proteins.
A
frameshift mutation results from an insertion or deletion of a number of nucleotides that is not a multiple of three. The change in reading frame alters every amino acid after the point of the mutation and results in a nonfunctional protein.
Spontaneous mutations occur through DNA replication errors, whereas
induced mutations occur through exposure to a
mutagen .
Mutagenic agents are frequently carcinogenic but not always. However, nearly all carcinogens are mutagenic.
Chemical mutagens include base analogs and chemicals that modify existing bases. In both cases, mutations are introduced after several rounds of DNA replication.
Ionizing radiation, such as X-rays and γ-rays, leads to breakage of the phosphodiester backbone of DNA and can also chemically modify bases to alter their base-pairing rules.
Nonionizing radiation like ultraviolet light may introduce pyrimidine (thymine) dimers, which, during DNA replication and transcription, may introduce frameshift or point mutations.
Cells have mechanisms to repair naturally occurring mutations. DNA polymerase has proofreading activity. Mismatch repair is a process to repair incorrectly incorporated bases after DNA replication has been completed.
Pyrimidine dimers can also be repaired. In
nucleotide excision repair (dark repair) , enzymes recognize the distortion introduced by the pyrimidine dimer and replace the damaged strand with the correct bases, using the undamaged DNA strand as a template. Bacteria and other organisms may also use
direct repair , in which the photolyase enzyme, in the presence of visible light, breaks apart the pyrimidines.
Through comparison of growth on the complete plate and lack of growth on media lacking specific nutrients, specific loss-of-function mutants called
auxotrophs can be identified.
The
Ames test is an inexpensive method that uses auxotrophic bacteria to measure mutagenicity of a chemical compound. Mutagenicity is an indicator of carcinogenic potential.
Fill in the blank
A chemical mutagen that is structurally similar to a nucleotide but has different base-pairing rules is called a ________.
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